1. Field of the Invention
The present invention relates to a high pressure discharge lamp light source device and particularly to a high pressure discharge lamp light source device which is capable of a stable lighting even if the lamp power is reduced to 40 to 70% of the nominal electric power consumption, and which, at the same time, can avoid wear of the electrodes.
2. Description of Related Art
High pressure discharge lamps, wherein at least 0.20 mg/mm2 mercury are enclosed in the interior of the light emission tube are known. Among projector devices in which such high pressure discharge lamp light source devices are suitably used, projectors employing a so-called ‘eco mode’ which uses less electric power than the nominal electric power have become common. The known ‘eco mode’ is a mode using electric power which has been suppressed to an amount of approximately 70 to 80% of the nominal electric power. It is, in other words, a mode of ‘lighting with low electric power’.
During the duration of such an eco mode the temperature of the electrode tip ends decreases because of the constraint of the electric power applied to the electrode tip ends, the position of the arc becomes unstable, and the occurrence of flicker is promoted. Techniques for the suppression of flicker in the above mentioned lamps are known, for example, from JP-A-2006-059790 and corresponding US 2005/0206326 A1 and JP-A-2008-509518 and corresponding US 2009/0009095 A1, which, however, do not improve the flicker during this low electric power lighting mode.
The technique stated in JP-A-2006-059790 and corresponding US 2005/0206326 A1 is characterized by the lighting frequency in the alternating current lighting mode. The frequency is controlled and a projection of the electrode tip end is formed projecting at the electrode tip end, and the arc is stabilized by rendering this projection the starting point of the arc. For the arc stabilization it is necessary that the size of the projection of the electrode tip end is controlled to a desired state according to the electric power, and one of the control methods is to change the drive frequency.
It is known that the projection becomes thick at a low frequency and thin at a high frequency, but the projection cannot be maintained by only repeating the waveform. With regard to this problem, the lighting frequency at which the maintenance and the stabilization of the projection can be accomplished is examined in the technique of JP-A-2006-059790 and corresponding US 2005/0206326 A1, and a stabilization of the arc is expected. By means of the above mentioned technique it was possible to maintain the arc stable even in a low electric power lighting mode such as the known eco mode.
In recent years, the needs have increased for projector devices to utilize a ‘brightness adjustment mode’ which employs a dimming function using a reduction of the lamp current according to the screen and an increase of the contrast, or a ‘super-eco mode’ using a further decrease of the electric power. Because of this situation, the lamp power is further constrained at the time of low electric power, and concretely a reduction of up to 40 to 70% of the nominal power consumption is necessary. But when the electric power is such decreased and reduced to less than 70% of the nominal power lighting, the arc becomes unstable even if measures are taken using generally known techniques, and before long flicker occurs.
The reason is explained in the following with regard to FIG. 14. In FIG. 14, 101 and 102 are respectively spherical parts of electrodes provided in the light emission tube of a high pressure discharge lamp, 101a and 102a are projections formed at the tip ends of the spherical parts, and 103 is the arc.
When lighting with the nominal electric power, the arc 103 is maintained by means of projection portions 101a, 102a with a relatively small volume provided beforehand at the tip ends of the spherical parts 101, 102 as shown in FIG. 14(a). At this time, the size of the projections 101a, 102a and the size of the electric power are balanced, the electrode tip end parts are maintained at a high temperature and the electron release is rendered easy.
When using the low electric power lighting mode with at most 70% of the nominal electric power, the temperature of the tip end parts decreases with the decrease of the electric power. Therefore, the electrode tip end temperature becomes too low with this size of the projections, a stable thermal electron release becomes impossible and it becomes impossible to maintain the arc 103 for a long time.
When the low temperature state of the electrode tip ends continues, the starting point of the arc 103 at the projection tip end moves with a short temporal cycle and, when having moved, forms another small projection (secondary projection). Resulting from the repetition of the movement of the arc 103 and the forming of a projection, a plurality of secondary projections is formed, and as shown in FIGS. 14(b) and (c), the arc 103 moves between the secondary projections.
When this situation occurs, the time of stay of the starting point of the arc at these secondary projections becomes long and the starting point of the arc moves between the secondary projections with a long temporal cycle. The reason for the time of stay of the starting point of the arc at a secondary projection becoming long is that the secondary projection reaches a high temperature because of its small size and a thermal electron release becomes possible. But when the size is too small, it cannot be maintained for a long time and wears off by evaporation, and when the distance (arc gap) between another secondary projection and the opposing electrode tip end becomes smaller, the starting point of the arc moves there.
The forming of secondary projections, the repetition of the forming and disappearance, the impossibility to maintain the arc at a specified position, and the movement of the starting point appear as flicker. Generally, humans do not perceive light fluctuations with a short cycle (>50 Hz), but light fluctuations with a long cycle (<50 Hz) are perceived. When the flicker of the lamp becomes significant, a flicker in the image projected to the screen appears, which is felt as unpleasant.
When a low electric power lighting mode with at most 70% of the nominal electric power is used, as stated above the temperature of the tip end part decreases together with the decrease of the electric power, the arc cannot be maintained at a constant position, the starting point moves, and flicker occurs.